Patent No. 2 557 985 relates to a deflector comprising a plurality of distinct crystals of paratellurite through which there passes a single polarized beam comprising a plurality of rays at different wavelengths. Ultrasound is applied to each of these crystals which then modulate respective ones of said rays as desired. When there are three such rays and three such crystals, the rays advantageously have wavelengths corresponding to the colors red, green, and blue so as to be able to form color images when the three modulated rays are combined. The modulated rays leave the crystal in succession along three respective directions which are not only very close to one another but which are also substantially parallel.
Patent No. 2 575 840 relates to a first improvement in which the three crystals are replaced by a single crystal having a special cut and which has ultrasonic signals applied thereto at points which are spaced apart. As a result, a suitably polarized incident beam emits a respective modulated emerging ray in relation to each of the spaced-apart points. The three emerging rays are likewise very close to one another and substantially parallel.
When going into greater detail, it is observed that the rays cannot be exactly parallel since their deflection in each crystal, or in the sole crystal, depends both on the wavelength .lambda. of the incident beam and on the acoustic frequency F injected into the crystal. For each of these rays, the deflection is proportional to .lambda.F. Consequently, although they appear to be substantially parallel, the various emerging rays diverge. There is an angle of about ten minutes of arc between the blue ray and the green ray, and the divergence between the red ray and the blue ray is thirty minutes of arc.
In order to provide a proper color image by superposing three prime color rays, it is necessary for the modulated rays to be strictly collinear.
The main aim of the present invention is therefore to improve the polychromatic acousto-optical deflector of the above-mentioned patents so that the modulated emerging rays are caused to be collinear when they leave the deflector.